This invention generally relates to phosphinic acids and more particularly to aromatic sulfonyl substitute phosphinic acids.
Polymeric metal dialkyl, diaryl and arylalkyl-phosphinates have been discussed in the literature. For instance, see "polymeric Metal Phosphinates", B. P. Block, Inorg. Macromol. Rev. 1 (1970) 115-125. Of particular interest are the polymers containing tetrahedral zinc (II) centers and symmtrical bridging O,O'-phosphinate groups in polymeric structures, such as ##STR4## wherein R and R' may be alkyl or aryl groups.
The zinc (II) and phosphinates groups are strong electrostatic centers. Strong electrostatic attraction between centers on adjacent polymer chains gives the dialkyl-, diaryl- and arylalkyl- zinc phosphinate polymers the properties of three dimensional polymers having crosslinkage between chains. As a result, the polymer chains do not easily slide over each other, causing the polymers to form brittle rather than flexible coatings. For instance, the diphenyl- and dimethyl- zinc phosphinate polymers form very brittle coatings. If the aryl and alkyl side groups are large enough to insulate the electrostatic centers on adjacent chains from each other, the polymer coatings will be flexible. For example, zinc phosphinate polymers in which the side groups are alkyl of 8 carbon atoms have a flexibility comparable to polyethylene. Unfortunately, polymers having these large aryl or alkyl side chains have poor thermal stability. The problem is therefore to develop polymers which are both flexible and thermally stable.
In addition to being flexible and thermally stable, the zinc (II) bis(phosphinate) polymers should be melt and/or solution processable. The symmetric diaryl or dialkyl zinc (II) bis(phosphinate) polymers, such as the diphenyl- or dimethyl-, have high energies of crystalization. As a result, these symetric polymers have very high melting points at which they usually decompose rather than melt; they also are insoluble in common solvents.